Process and apparatus for elongating of an optical fibre preform

ABSTRACT

A process and apparatus for elongating an optical fibre preform includes heating the preform so as to soften one region thereof; elongating the preform by submitting the preform to a traction; determining, during the step of elongating, the preform diameter in at least one measuring point along the preform; and controlling the step of elongating on the basis of the determined diameter. During the step of elongating, at least a geometrical parameter of the preform is measured, and the position of said diameter measuring point is controlled according to the measured geometrical parameter. Measuring at least a geometrical parameter of the preform may be accomplished by determining the profile of at least a portion of the softened region, e.g., an image of the neck region profile.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP2002/008479, filed Jul. 30, 2002, the content of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the manufacturing of opticalfibers, and particularly to processes for forming glass preforms for theoptical fibers. Specifically, the invention concerns a process and anapparatus for the elongation of an optical fiber preform.

2. Description of the Related Art

Several processes are known for making glass preforms for drawingoptical fibers. Such processes include the modified chemical vapourdeposition (MCVD) process, the outside vapour deposition (OVD) processand the vapour axial deposition (VAD) process.

Many of the known processes for making the preforms include a stage,called elongation, in which a vitrified preform rod, formed according todifferent techniques depending on the specific manufacturing process, issubmitted to reduction in diameter to obtain a preform of prescribedfinal diameter. To this purpose, the vitrified preform rod is heated, ina furnace or by means of a burner, up to the softening temperature. Thepreform rod is then stretched so as to reduce the diameter thereof inthe softened region, referred to as “neck”. The vitrified preform rodmay have a central hole that, during the elongation stage, may collapse.

Several solutions are known for stretching the preform. According tosome solutions, both ends of the preform are moved, while the heatedregion of the preform is kept steady. In this case, the preform isgenerally elongated along a vertical axis (“vertical elongationprocess”), and it is attached at the upper end thereof to a feeder; thefeeder sustains the preform and feeds it to a furnace. At the bottomend, downstream the furnace, the preform is attached to a puller,providing the traction force necessary to stretch the preform.

Within the furnace, the preform is heated up to the softeningtemperature. The puller imparts a translation speed higher than the feedrate of the feeder, thereby the softened region of the preform isstretched. The outer diameter of the preform in the softened region isthus reduced and, if present, the preform central hole may collapse.Optionally, the feeder and the puller may also impart to the preform arotation about its axis.

According to other solutions, one end of the preform is kept steady andthe other end is moved, while the heated region is moved in a directionsame as or opposite to the moving end of the preform. In this solutions,the preform is generally elongated along a horizontal axis (“horizontalelongation process”). The preform is heated by means of a horizontallymovable heater, e.g. a burner mounted on a carriage. The preform endsare attached to mandrels of a horizontal lathe: one mandrel is keptsteady, while the other is moved horizontally. The translation speeds ofthe movable mandrel and the heater determine the final diameter of thepreform. Also in this case, the preform may be rotated about its axis.

Irrespective of the specific solution adopted for stretching thepreform, the main objective of the preform elongation stage is that ofobtaining rods of a prescribed diameter, to be submitted to subsequentprocessing up to the drawing of optical fibers. It is therefore ofparamount importance to monitor the preform diameter during theelongation stage.

Various techniques have been proposed for monitoring the preformdiameter during the elongation stage. Generally speaking, all thesetechniques call for measuring the preform diameter in a limited numberof discrete points (one, two or three points) along the preform axis,particularly along the neck, for example by means of laser-basedinstruments; the measured diameter or diameters are typically comparedto predetermined diameter values, and the feed rate of the feeder and/orthe speed of the puller, or the speed of the movable mandrel and/or thespeed of the heater, depending on the solution adopted for stretchingthe preform, are controlled accordingly. For example, assuming that themeasured diameter is higher than the target diameter, the speed of themovable mandrel is increased, and vice versa.

Techniques providing for measuring the preform diameter in oneprescribed point along the preform axis are for example described in JP57092534, JP 62108743, JP 61014149, U.S. Pat. No. 5,755,849 and U.S.Pat. No. 5,942,019. JP 5147971, U.S. Pat. No. 6,178,778 and JP 8091861are examples of prior art documents describing the measurement of thepreform diameter in two or three discrete points along the preform axis.

In particular, U.S. Pat. No. 5,942,019, in relation to the elongation ofpreforms by means of a furnace, underlines the importance of setting theposition for measuring the outer diameter of the taper portion (i.e.,the neck). Summarising, in that document it is observed that in the casewhere the outer diameter measuring position is disposed near the upperend of the taper portion, i.e., near the heater, even when the movingspeeds of the chucks are controlled to keep the outer diameter of theupper end of the taper portion constant, the outer diameter may bevaried at the taper portion, thereby the outer diameter of the elongatedbody may become uneven and fluctuate. On the other hand, in the casewhere the outer diameter measuring position is disposed near the lowerend of the taper portion, since the glass preform has almost been cooledat this position and its viscosity has been quite large to be elongated,even if a fluctuation in the outer diameter is detected, it can hardlybe corrected. Still according to U.S. Pat. No. 5,942,019, the optimalouter diameter measuring position, which varies depending on the outerdiameter of the glass preform before elongating, the outer diameter ofthe elongated body, the heater temperature, the inner diameter of thefurnace core tube and the like, is to be determined experimentally.

According to the Applicant, determining the optimal position of thediameter measurement point by means of experiments is not satisfactoryfrom an industrial application viewpoint.

The Applicant has moreover noticed that prior art methods provides formeasuring the diameter in one, two or three prefixed points, inparticular in points that have no correlation with the actual geometryof the neck of the preform being elongated, and that because of thedependency of the neck length and shape on several process parameters,such as the initial and final diameter of the preform, the processoperating speeds, the temperature profiles, the speed of rotation of thepreform, if provided, the diameter of the preform central hole, ifpresent, and the internal pressure, the neck geometry may vary fromprocess to process and even in the course of a single elongationprocess. The Applicant has found that, for this reason, measuring theneck diameter in prefixed points, which are not correlated to thegeometry of the neck, does not allow a precise control of the preformfinal diameter.

SUMMARY OF THE INVENTION

Based on these considerations, the Applicant has devised a new opticalfiber preform elongation process, which comprises, during the step ofelongating the preform, measuring at least a geometrical parameter ofthe preform being elongated and controlling the position of a preformdiameter measuring point according to the measured geometricalparameter.

In particular, measuring the at least a geometrical parameter maycomprise detecting a profile of at least a portion of the preformsoftened region, for example capturing a digital image of the at least aportion of the softened region.

According to a first aspect of the present invention, there is providedan optical fiber preform elongation process.

Briefly stated, the process comprises heating the preform so as tosoften one region thereof; elongating the preform by submitting thepreform to a traction; determining, during the step of elongating, thepreform diameter in at least one measuring point along the preform; andcontrolling the step of elongating on the basis of the determineddiameter.

The process further comprises measuring, during the step of elongating,at least a geometrical parameter of the preform; and controlling, duringthe step of elongating, the position of said diameter measuring pointaccording to the measured geometrical parameter.

In an embodiment of the invention, measuring at least a geometricalparameter of the preform comprises determining the profile of at least aportion of the softened region.

In particular, measuring at least a geometrical parameter of the preformmay comprise detecting, from said determined profile, at least one amonga softened region starting point and a softened region final point, andcontrolling the position of said measuring point comprises choosing adiameter measuring point located at a predetermined distance from oneamong the softened region starting point and the softened region finalpoint.

Measuring at least a geometrical parameter of the preform may furthercomprise detecting, from said determined profile, the length of thesoftened region, and said predetermined distance may be a predeterminedpercentage of said length.

In an embodiment of the invention, the profile is determined bydetecting a predetermined number of points along the profile of thepreform and interpolating said points.

Preferably, determining the profile comprises capturing a digital imageof the at least a portion of the softened region.

In an embodiment of the invention, controlling the step of elongatingcomprises comparing the determined diameter with a target diameter.

The process may comprise feeding the preform to a furnace at a firstspeed, and submitting the preform to a traction by pulling the preformout of the furnace at a second speed; controlling the step of elongatingcomprises controlling at least one among the first speed and the secondspeed.

Alternatively, the process may comprise exposing the preform to a heatermovable along a preform axis at a first speed, and applying a tractionby pulling at least one end of the preform at a second speed;controlling the step of elongating comprises controlling at least oneamong the first speed and the second speed.

According to a second aspect of the present invention, there is providedan optical fiber preform elongation process.

Summarising, the process according to this second aspect of theinvention comprises heating the preform so as to soften one regionthereof; elongating the preform by submitting the preform to a traction;determining at least a geometrical parameter of the preform; andcontrolling the step of elongating on the basis of the detectedgeometrical parameter.

Determining at least a geometrical parameter comprises detecting theprofile of at least a portion of the softened region.

In particular, detecting the profile comprises detecting a predeterminednumber of points along the profile of the preform and interpolating saidpoints.

Preferably, detecting the profile comprises capturing a digital image ofthe at least a portion of the softened region.

In an embodiment of the invention, determining at least a geometricalparameter comprises determining the preform diameter in a measuringpoint of the softened region, and controlling the step of elongatingcomprises comparing the determined diameter with a target diameter.

In particular, determining the preform diameter comprises controllingthe position of the measuring point according to said detected profile.

The process may further comprise controlling the target diameteraccording to said detected profile.

In an embodiment of the invention, the preform diameter is determinedfrom said detected profile.

In particular, determining at least a geometrical parameter comprisesdetermining, from said detected profile, at least one among a softenedregion starting point and a softened region final point, and controllingthe position of the measuring point comprises choosing a measuring pointlocated at a predetermined distance from one among the softened regionstarting point and the softened region final point.

In an embodiment of the invention, measuring at least a geometricalparameter of the preform further comprises detecting, from saiddetermined profile, the length of the softened region, and saidpredetermined distance is a predetermined percentage of said length.

According to a third aspect of the present invention, there is provideda process for manufacturing an optical fiber.

In brief, the process according to this third aspect of the inventioncomprises producing a glass preform and drawing the glass preform intoan optical fiber.

Producing the glass preform comprises the steps of heating anintermediate preform so as to soften one region thereof; elongating theintermediate preform by submitting the intermediate preform to atraction; detecting, during the step of elongating, the preform diameterin at least one measuring point along the intermediate preform; andcontrolling the step of elongating on the basis of the detecteddiameter.

The process further comprises measuring, during the step of elongating,at least a geometrical parameter of the preform; and varying, during thestep of elongating, said measuring point according to the measuredgeometrical parameter.

According to a fourth aspect of the present invention, an apparatus forelongating an optical fiber preform is provided.

Briefly stated, the apparatus comprises a monitoring device forobtaining information on geometrical parameters of the preform beingelongated; and a control device for controlling elongation processparameters using the preform geometrical parameters information.

The monitoring device comprises an image capturing device for obtaininga profile of at least a portion of a softened region of the preform; anda processing device for analysing the profile for extracting informationon the preform geometrical parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bemade apparent by the following detailed description of an embodimentthereof, provided merely by way of non-limitative example, which will bemade in connection with the attached drawings, wherein:

FIG. 1 is a pictorial view of a preform elongation apparatus accordingto an embodiment of the present invention;

FIG. 2 is a simplified flowchart of a preform elongation control methodaccording to an embodiment of the present invention;

FIG. 3 schematically shows a profile of the preform softened region (thepreform “neck”), resulting from an image processing procedure of thepreform elongation control procedure;

FIG. 4 is a simplified flowchart of a preform neck profile analysisprocedure of the preform elongation control procedure;

FIGS. 5A, 5B and 5C are simplified flowcharts of three alternativeembodiments of a procedure for determining a diameter measurement pointalong the preform neck;

FIG. 6 is an image of a preform neck captured during a neck profileacquisition experimental trial conducted by the Applicant using ablack-and-white digital camera;

FIG. 7 shows the image of the neck after a first image processing stepinvolving converting the grey level of any pixel into black or whiteaccording to a predetermined threshold;

FIG. 8 shows the image of the neck after a second image processing stepinvolving filtering the image for eliminating disturbs;

FIG. 9 shows the neck profile extracted from the processed image of FIG.8;

FIG. 10 shows the neck profile obtained interpolating the neck profileof FIG. 9;

FIG. 11 shows the profiles of the neck of two preforms elongatedaccording to elongation processes with different operating parameters;and

FIG. 12 shows the difference in the diameters of the two preform necksmeasured according to a conventional technique and according to twotechniques according to two embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, FIG. 1 is a pictorial, very schematicrepresentation of an apparatus for elongating an optical fiber preform,according to an embodiment of the present invention.

The elongation apparatus, identified globally by 101, comprises ahorizontal lathe 105, having a steady mandrel 107 a and a movablemandrel 107 b, spaced apart horizontally.

A glass preform 109 is attached at a first and second ends 109 a, 109 bthereof to the steady and movable mandrel 107 a, 107 b, respectively. Tothis purpose, the first and second preform ends 109 a and 109 b may beprovided with conventional handles (not shown in the drawing). Thepreform 109 extends therefore between the steady and the movablemandrels 107 a and 107 b in such a way that a longitudinal axis 109 c ofthe preform lies in a substantially horizontal plane.

A movable carriage 111 is provided, movable on a horizontal plane alongthe direction of the preform axis 109 c. The carriage 111 carries aburner 113, oriented towards the preform for heating a region thereof upto a softening temperature, and a visual image capturing system 115,comprising for example a high-definition digital camera 117, adapted tocapture an image of at least a portion of a softened region 109 d (theneck) of the preform 109, intermediate between an already elongatedpreform section 109 e, the diameter of which has already been reduced tothe desired final diameter, and a non-elongated preform section 109 f,the diameter of which is equal to the preform initial diameter. Inparticular, the image capturing system is adapted to capture an image ofat least a final portion of the neck 109 d, proximate to the alreadyelongated preform section 109 e. The image capturing system 115 mayinclude a light source 125, adapted to illuminate the preform region 109d of which the image is to be captured. Where possible, a suitablebackground element 127 is provided along the preform at the oppositeside of the image capturing system 115, to ensure sufficient contrast ofthe neck image with respect to the image background. Alternatively, thelight source is located opposite to the image capturing system 115.

Preferably, the digital camera 117 is provided with a filter or filtersfor improving the quality of the captured images; in particular,infrared filters or polarizing filters may be used. In case amonochromatic light source is used, the filters are selective towardsthe light source wavelength.

The image capturing system 115 feeds a processing and control unit 119,for example a personal computer with installed a suitable imageprocessing software. The processing and control unit 119 processesvisual images captured by the image capturing system 115, analyses theprocessed image and controls one or both of two drive units 121, 123(e.g., motors) driving the carriage 113 and the movable mandrel 107 b,so as to set the respective translation speeds, for controlling thediameter of the elongated preform.

It is pointed out that the type of apparatus employed for elongating thepreform is not to be intended as limitative for the present invention.Instead of the exemplary apparatus described before, a verticalelongation apparatus comprising a preform feeder, a furnace and apreform puller could for example be employed. In this case, a camera canbe associated with the furnace in such a way as to be able to capture animage of the neck or of a predetermined portion thereof.

FIG. 2 shows a simplified flowchart illustrating the main steps of apreform elongation control procedure 201.

In a first step (block 203) an image of the neck 109 d (or at least of aportion thereof, preferably the portion proximate to the preform section109 e) is captured by the image capturing system 115.

The captured image is fed to the processing and control unit 119 (block205). By means of the image processing software, the processing andcontrol unit 119 processes the captured image; in particular, the imageprocessing allows separating the significant parts of the image (thepreform neck) from the non-significant image parts, such as thebackground, or disturbs, and extracting a continuous profile of thepreform neck, or of the at least a portion thereof (block 207).

An analysis of the neck profile that is obtained from the imageprocessing procedure is then performed by the image processing andcontrol unit 119 to extract geometrical parameters of the neck from theobtained profile (block 209), and the results of the neck profileanalysis are exploited to control the speed of the carriage 111 (andthus of the burner 113) and/or of the movable mandrel 107 b (block 211),thus implementing a control loop. For example, by means of the analysisof the neck profile, significant parameters can be extracted that areused for controlling the elongation stage.

The procedure goes on until the whole preform has been elongated (block213).

Concerning the image processing (block 207), it is observed that theimage captured by a digital camera is represented by a matrix of pictureelements (pixels); a pixel is the elemental unit in which the capturedimage is subdivided. The number of pixels in the matrix depends on theprecision (resolution) of the digital camera: for example, a digitalcamera having a resolution of 1,000×1,000 pixels provides imagesrepresented by a matrix of 1,000,000 pixels (1,000 along the horizontaldirection and 1,000 along the vertical one), each pixel corresponding toa respective area of the scene.

Each pixel has associated therewith an information relating to thecolour of the corresponding area of the scene; in a black-and-whitedigital camera, the colour is represented by a grey level in a greylevel scale whose two extremes corresponds to the black and the white,respectively. The colour or grey level information associated with eachpixel is encoded in a digital code. For example, a digital camera inwhich each grey level is expressed by means of a 12-bit digital codeallows discriminating among 4096 different grey levels, and to eachpixel there is associated a 12-bit binary coded integer of valuecomprised between 2⁰ (e.g., corresponding to the black) and 2¹²(corresponding to the white), identifying the level of grey of thatpixel.

Processing the image corresponds to processing the matrix of pixels andthe associated binary codes representing the respective grey levels.

Any commercially available image processing software can in principle beexploited; alternatively, a custom designed image processing softwaremay be used. Independently of the image processing software adopted, thebasic actions preformed in the image processing step are describedherein below.

Before trying to extract the neck profile from the captured image, theimage is preferably filtered to suppress noise and disturbs. Any knownimage filtering algorithm can be adopted.

After having filtered the captured image, the contour of the preformneck is identified. This step provides for identifying, among all thepixels in the pixel matrix, the pixels that belong to the neck contour.One possible algorithm for achieving this result provides forestablishing a prescribed threshold grey level. Then, the pixels in thematrix having a grey level lower (alternatively, higher) than theprescribed threshold are identified and declared to belong to the neckcontour. An alternative algorithm may identify the pixels belonging tothe neck contour by comparing the grey level of pixels adjacent to eachother, and declaring that all the pixels whose grey levels differ fromthe grey levels of the previous pixels of at least a prescribed valuebelong to the neck profile. Other pattern recognition algorithms may beused, implemented by commercially available computer programs.

Optionally, once the pixels belonging to the neck contour have beenidentified, an image interpolation can be carried out to increase theprecision of the image. To this purpose, known image interpolationalgorithms can be exploited. The precision of the image can thus beincreased beyond that of the single pixel.

FIG. 3 schematically shows a continuous neck profile that is obtainedfrom the captured image after the captured image has been processed;actually, the neck profile obtained from the captured image is normallynot continuous (being obtained from pixels), but the precision of theimage capturing system and the image processing software can be suchthat the profile can, at all effects, be regarded as substantiallycontinuous. The rectangle 301 represents the image area, and the curves303 within the rectangle 301 identifies the neck profile. The conversionfactor from pixels to metric units can be determined rather easilyprovided that the area of the scene (L1 and L2) in metric units isknown. Alternatively, or in addition, a metric reference can beintroduced in the area of scene. In still another embodiment, aconventional diameter measurement device can be provided upstream and/ordownstream the image capturing system; the diameter value measured bysuch device or devices can be used as a set point for thepixel-to-metric unit conversion (the neck diameter, in metric units,measured by such diameter measurement device is compared to the neckdiameter in pixel obtained from the captured image, and thepixel-to-metric unit conversion factor is established).

It can be appreciated that the information that can be derived from theobtained continuous profile of the neck, or even of a portion thereof,for example the portion of the neck proximate to the already elongatedpreform section, is substantially richer than that derivable from asimple measurement of the preform diameter in one or more discretepoints along the neck.

In particular, obtaining the continuous profile of the neck allowsovercoming the problems, affecting the conventional elongation controltechniques, inherent to the choice of the point or points in which tomeasure the preform diameter.

For example, the analysis of the obtained neck profile allowsdetermining in real time the neck starting point (x1 in FIG. 3) and theneck final point (x2 in FIG 3), the neck length (|x2−x1|), the diameterof the preform (D in FIG. 3) at every point along the neck. The analysisof the neck profile may even allow deriving a mathematical formuladescribing analytically the neck profile.

From a practical viewpoint, the choice of the image capturing systemshall take into account two important parameters, that are theresolution of the captured images and the scan frequency.

As mentioned, the image resolution is an indication of the precision ofthe image capturing system.

The level of detail in the captured image depends however not only onthe resolution of the image capturing system, but also on the area ofthe scene. By way of example, if the above mentioned 1,000×1,000 pixelsdigital camera is used to get an image of a scene having an area of100×100 mm, each pixel represents an area of 0.1×0.1 mm of the scene.

Image interpolation processes may allow increasing the precision of thecaptured image, enabling to detect details below the dimension of thepixel.

The scan frequency is an indication of the number of times per secondthe image is captured. In order to constantly monitor the neck shapeduring the preform elongation stage, a sufficiently high scan frequencyis required. However, there is a trade off between the scan frequencyand the precision of the captured image: the higher the image precision,the higher the resolution of the image capturing device, the higher thenumber of pixels, the more bits need to be transferred, the lower thescanning frequency.

Commercially available digital cameras feature resolutions in pixels ofthe order of 512×512, 1000×1000, 1300×1000, 2000×2000 and 4000×4000,with decreasing scan frequencies (from 100–200 frame per seconds—fps—to0.5 fps). Assuming again that an image of a scene of area 100×100 mm isto be captured, the precision of the image increases with the increasein resolution, but at the expense of a decrease in the scan frequencies.

A satisfactory trade off is considered reached adopting a commerciallyavailable digital camera having a resolution of 1300×1000 pixels with ascan frequency of about 15 fps; this digital camera allows obtainingimages of a scene of area 100×100 mm with a precision at least equal to0.076 mm, and is considered suitable for acquiring images of the preformneck profile that can be used for finely controlling the elongationprocess.

If a higher precision is desired, without impacting the scan frequency,custom designed image capturing systems can be exploited.

It is pointed out that although a black-and-white digital camera isadvantageous in terms of image definition and scan frequency, ananalogue camera, and particularly a colour camera may be used. Othertypes of visual image capturing systems may be envisaged, for examplelaser-based instruments moved longitudinally to the neck at a suitablespeed.

It is observed that, by using suitable interpolation algorithms, acontinuous profile may also be reconstructed starting from a limitednumber of points, much lower that the number of points provided by acamera.

The high information content inherent to the availability of thecontinuous neck profile allows implementing a variety of techniquesenabling an effective control of the preform elongation stage.

An exemplary embodiment of control technique according to the presentinvention will be now described, referring to the simplified flowchartof FIG. 4 that schematically shows the neck profile analysis procedureof block 209 in FIG. 2.

Firstly, the neck profile is analysed to determine the neck startingpoint x1 and/or the neck final point x2 (block 401). The neck startingpoint x1 can be determined for example by determining the point alongthe neck profile at which the diameter of the preform for the first timefalls below the preform initial diameter of a prescribed threshold. Incase an analytical formula describing the neck profile is extrapolatedfrom the neck profile, the neck starting point x1 can be determined bymonitoring the first derivative. The neck final point x2 can bedetermined in a similar way.

Based on the determined neck starting and final points x1 and x2, thepoint x3 along the neck is determined at which the neck diameter D(x3)is to be calculated (block 403).

FIGS. 5A, 5B and 5C are simplified flowcharts of three possibleprocedures for determining the optimal point x3 at which the neckdiameter is to be calculated so as to enable a precise control of thepreform diameter.

According to a first procedure, shown in FIG. 5A, the point x3 at whichthe neck diameter is to be calculated is fixed with respect to the neckfinal point x2; once the neck final point x2 is determined, the point x3is obtained subtracting from the point x2 a prescribed constant K1(block 501 a).

According to a second procedure, shown in FIG. 5B, the point x3 is fixedwith respect to the neck starting point x1; once the neck starting pointx1 is determined, the point x3 is obtained adding to the point x2 aprescribed constant K2 (block 501 b).

According to a third procedure, shown in FIG. 5C, the length of the neckis first calculated (block 501 c) by subtracting x1 to x2, and the pointx3 is determined by calculating a distance from the neck starting pointx1 equal to a prescribed percentage K3 of the neck length |x2-x1| (block503 c). Alternatively, the point x3 can be determined as a distance fromthe neck final point x2 equal to a prescribed percentage of the necklength.

It can be appreciated that since points x1 and x2 are subjected tovariations during the process of elongation, the point x3 varies aswell.

It is pointed out that the choice of the reference for determining thepoint at which the preform diameter is to be calculated depends on thetype of control algorithm. For example, if it is preferred to monitorthe preform diameter in a region proximate to the final section of theneck, it is preferable to calculate the preform diameter at a pointlocated at a prescribed distance from the neck final point (determinedby inspection of the neck profile). It is also possible to monitor thepreform diameter in two or more positions, one proximate to the neckfinal point and one proximate to the neck starting point; in this case,the diameter of the preform may be calculated at a point located at aprescribed distance from the neck final point, and at a point located ata prescribed distance from the neck starting point, both the neck finalpoint and the neck starting point being derived by inspection of theneck profile.

After having determined the point x3 at which the neck diameter is to becalculated, the diameter D(x3) at such point along the neck iscalculated from the neck profile (block 405).

After having calculated the neck diameter D(x3) in the prescribed pointx3, the calculated diameter D(x3) is compared to a prescribed, targetdiameter, stored in the processing and control unit 119, and thedeviation of the calculated diameter from the target diameter isdetermined (block 407). Such a deviation is used to control the speedsof the movable mandrel 107 b and of the carriage 111 carrying the burner113 (block 211 in FIG. 2). For example, if the calculated diameter ishigher than the target diameter, the speed of the movable mandrel isincreased, and vice versa.

Summarising, on the basis of the continuous profile of the neck obtainedafter processing the image captured by the image capturing device 115,the diameter of the preform is determined (from the analysis of the neckprofile) in at least one point (identified as x3 in FIG 3) whoseposition along the neck is not fixed with respect to the heated regionof the preform, in particular to the heating element (furnace orburner), as in the known techniques, but varies depending on the neckgeometry, which is derived from the analysis of the neck profile. Forexample, the diameter of the neck is calculated at a point, along theneck profile, which is fixed with respect to the neck starting point orto the neck final point, determined in turn by analysing the neckprofile.

Compared to the known techniques, in which the position of the point orpoints at which the preform diameter is measured is fixed with respectto the heated region of the preform, monitoring the neck profile at apoint or points whose position is not fixed a priori, but depends on theactual neck geometry allows reducing the problems inherent to thevariation of the neck shape and length due to varying processparameters.

In an alternative embodiment, the diameter of the preform is calculatedfrom the obtained profile in one or more prefixed points, whose positiondoes not vary with the geometry of the neck. The information on the neckgeometry derived from the analysis of the acquired image of the neck isexploited to vary the target value or values according to the geometryof the neck.

Experimental Results

The Applicant has conducted some experiments, which are reportedhereinbelow.

Neck Profile Acquisition

FIG. 6 shows the image of a preform neck captured using ablack-and-white digital camera with 256 grey levels (having valuesranging from 0 to 255), a resolution of 1360×1024 pixels and a scanfrequency of 9.5 fps, placed at approximately 60 cm from the preform.This image was fed to the processing and control unit 119, whichperformed an image processing. The neck has been obtained by means of avertical elongation process; in particular, the image has been takenstopping the elongation process and sliding the preform down thefurnace. The rectangle visible around the preform is a containing tubenormally associated with the lower portion of the furnace.

FIG. 7 shows the captured image after a first image processing stage. Inparticular, in order to identify the area of the image occupied by theneck, the pixels having a grey level lower than a prescribed thresholdvalue, in particular 210, have been converted to black pixels (greylevel value equal to 0), while the pixels having a grey level higherthan the prescribed threshold value have been converted to white pixels(grey level value equal to 256). It can be appreciated that in the imageresulting from this first processing stage the shape of the neck can beidentified rather clearly, albeit several disturb are present, havinghowever an area much smaller than that of interest. The disturbs wereeliminated from the image by means of a filtering process, suppressingall the white areas below a prescribed dimension threshold. The imageresulting from such filtering process is shown in FIG. 8.

Then, sections of the image of FIG. 8 were taken along each row of thepixel matrix (i.e., transversally to the preform axis), and the pixelslying on the contour of the neck were identified as those at whichtransition from black to white and from white to black occurs. In thisway, the neck profile depicted in FIG. 9 was determined; in thisdiagram, the scales of the axes are in millimeters, and the neck profileis turned of 90° compared to the image of FIG. 8.

FIG. 10 depicts the neck profile after an interpolation process; inparticular, the interpolation was carried out using polynomial functionsof the sixth order, that interpolated the data with a root mean squareerror equal to 0.9998.

Using a commercial microprocessor with a clock frequency of about 700MHz, the steps from the image capture to the interpolation lastapproximately 50 ms, corresponding to a maximum scan frequency ofapproximately 20 fps. The cost in term of computing time clearly dependson the image resolution and on the type of image processing algorithm.In case higher image resolutions or more complex image processingalgorithms are desired, more powerful microprocessors can be used.

Elongation Stage Control

The Applicant also conducted experimental trials to verify that, basedon the analysis of the acquired neck profile, a more precise control ofthe preform elongation stage can be achieved.

In particular, the Applicant executed two processes of elongation of twopreforms, using an apparatus as that pictorially shown in FIG. 1. Duringthe elongation stage, the movable mandrel 107 b was moved rightwards,while the carriage 111 carrying the burner 113 was moved leftwards. Itcan be shown that, for reasons of mass flow balance, the starting andfinal diameters D1 and D2 of the preform prior to and after theelongation, and the translation speeds V1 and V2 of the mandrel and theburner are related by the following equation:D1² *V1=D2²*(V1+V2)

The two elongation processes were carried out keeping fixed the initialand final diameters D1 and D2 at respective values of 20 mm and 15.3 mm,but varying the speeds V1 and V2. In particular, the first process wascarried out with V1=21 mm/min and V2=15 mm/min, while for the secondprocess the speeds were V1=32 mm/min and V2=23 mm/min. It is pointed outthat such a large difference in the speed between different processeswas deliberately introduced to magnify the effects of the change inprocess parameters on the preform elongation.

A digital camera similar to that used in the image acquisitionexperiment previously reported has been employed. The profiles of thepreform necks for the two processes were determined.

FIG. 11 reports the diameter of the preforms along the axes thereof inthe neck region, in the two elongation processes; these diameter valueswere obtained analysing the profile of the neck in the two cases. It canbe appreciated that despite the initial and final diameters are thesame, the diameter varies according to different laws in the twoprocesses, due to the difference in the speeds V1 and V2; in particular,the neck length, the neck starting point and the neck final point differin the two processes: in other words, the neck length and shape varydepending on the process parameters.

Should the preform diameter be measured at a point fixed with respect tothe burner, the difference in the diameters measured in the processescould be as large as 1.5 mm, without differences in the preform finaldiameter after the elongation. It may even happen that the diameter ismeasured at a point where the neck is not yet begun, or is alreadyterminated.

This confirms that measuring the preform diameter in one or morediscrete points at predetermined, fixed positions with respect to theburner (or, more generally, fixed with respect to the heated region ofthe preform) does not provide sufficient information for effectivelycontrolling the elongation stage.

The diagram in FIG. 12 reports the difference (D1−D2) of the preformdiameters along the preform axes in the two processes. In particular,curve A was obtained by calculating the difference (D1−D2) at pointslocated at a same longitudinal position within the heated region of thepreform; curve B was obtained calculating the diameter difference(D1−D2) at points located at a same distance from the beginning of theneck (which, as mentioned, varies in the two processes); curve C wasobtained calculating the difference (D1−D2) at points located at adistance from the beginning of the neck equal to a same, prescribedpercentage of the neck length, for example half the neck length.

It can be appreciated that by measuring the preform diameter at pointswhose position is not fixed with respect to the heated region, butvaries depending on the neck geometry as derived from the inspection ofthe neck profile, it is possible to reduce the errors inherent to thedifference in the neck geometry induced by disturbs or variation of theprocess parameters. While in the case of curve A the maximum difference(D1−D2) is higher than 1.5 mm, such a difference is lower than 0.8 mmand 0.5 mm in the case of curves C and B, respectively: a reduction ofmore than ⅔ can be achieved.

FIG. 11 shows that the geometry of the neck may vary significantly evenif the initial and final diameters are the same. In this condition, theconventional control techniques, detecting a deviation of the measureddiameter from the target diameter, would cause an undesired variation inthe final diameter.

FIG. 12 shows instead that the deviation from a target diameter can besignificantly reduced if the diameter of the preform is determined at apoint that is not fixed with respect to the heated region, but whoseposition varies according to the geometry of the neck, derived from theinspection of the acquired neck profile. In this way, undesiredvariations on the final diameter are greatly reduced.

Although the present invention has been disclosed and described by wayof some embodiments, it is apparent to those skilled in the art thatseveral modifications to the described embodiments, as well as otherembodiments of the present invention are possible without departing fromthe scope thereof as defined in the appended claims.

In particular, albeit in the detailed description that has been providedthe image capturing means were visual image capturing means, other typesof image capturing means may be exploited, for example operating in theinfrared spectrum.

The present invention can be applied to any process of manufacturing ofoptical fiber preforms that includes an elongation stage, for exampleMCVD, OVD and VAD processes.

1. An optical fiber preform elongation process, comprising: heating thepreform so as to soften one region thereof; elongating the preform bysubmitting the preform to a traction; determining, during the step ofelongating, the preform diameter in at least one measuring point alongthe preform; controlling the step of elongating on the basis of thedetermined diameter; measuring, during the step of elongating, at leasta geometrical parameter of the preform, the geometrical parameter beingdifferent than the at least one measuring point; and controlling, duringthe step of elongating, the position of said at least one measuringpoint according to the measured geometrical parameter.
 2. The processaccording to claim 1, wherein measuring at least a geometrical parameterof the preform comprises determining the profile of at least a portionof the softened region.
 3. The process according to claim 2, whereinmeasuring at least a geometrical parameter of the preform comprisesdetecting, from said determined profile, at least one among a softenedregion starting point and a softened region final point, and whereincontrolling the position of said measuring point comprises choosing adiameter measuring point located at a predetermined distance from oneamong the softened region starting point and the softened region finalpoint.
 4. The process according to claim 3, wherein measuring at least ageometrical parameter of the preform further comprises detecting, fromsaid determined profile, the length of the softened region, and whereinsaid predetermined distance is a predetermined percentage of saidlength.
 5. The process according to claim 2, wherein determining theprofile comprises detecting a predetermined number of points along theprofile of the preform and interpolating said points.
 6. The processaccording to claim 2, wherein determining the profile comprisescapturing a digital image of the at least a portion of the softenedregion.
 7. The process according to claim 1, wherein controlling thestep of elongating comprises comparing the determined diameter with atarget diameter.
 8. The process according to claim 1, wherein heatingthe preform comprises feeding the preform to a furnace at a first speed,and submitting the preform to a traction which comprises pulling thepreform out of the furnace at a second speed; and wherein controllingthe step of elongating comprises controlling at least one among thefirst speed and the second speed.
 9. The process according to claim 1,wherein heating the preform comprises exposing the preform to a heatermovable along a preform axis at a first speed, and applying a tractionwhich comprises pulling at least one end of the preform at a secondspeed, and wherein controlling the step of elongating comprisescontrolling at least one among the first speed and the second speed. 10.An optical fiber preform elongation process, comprising: heating thepreform so as to soften one region thereof; elongating the preform bysubmitting the preform to a traction; determining a preform diameter ata measuring point in the softened region; determining at least ageometrical parameter of the preform which comprises detecting theprofile of at least a portion of the softened region, the geometricalparameter being different than the measuring point in the softenedregion; controlling the step of elongating on the basis of thedetermined preform diameter; and controlling the position of themeasuring point according to the geometrical parameter.
 11. The processaccording to claim 10, wherein detecting the profile comprises detectinga predetermined number of points along the profile of the preform andinterpolating said points.
 12. The process according to claim 10,wherein detecting the profile comprises capturing a digital image of theat least a portion of the softened region.
 13. The process according toclaim 10, wherein controlling the step of elongating comprises comparingthe determined diameter with a target diameter.
 14. The processaccording to claim 13, further comprising controlling the targetdiameter according to said detected profile.
 15. The process accordingto claim 13, wherein the preform diameter is determined from saiddetected profile.
 16. The process according to claim 10, whereindetermining the preform diameter comprises controlling the position ofthe measuring point according to said detected profile.
 17. The processaccording to claim 10, wherein determining at least a geometricalparameter comprises determining, from said detected profile, at leastone among a softened region starting point and a softened region finalpoint, and wherein controlling the position of the measuring pointcomprises choosing a measuring point located at a predetermined distancefrom one among the softened region starting point and the softenedregion final point.
 18. The process according to claim 17, whereinmeasuring at least a geometrical parameter of the preform furthercomprises detecting, from said determined profile, the length of thesoftened region, and wherein said predetermined distance is apredetermined percentage of said length.
 19. A process for manufacturingan optical fiber, comprising producing a glass preform and drawing theglass preform into an optical fiber, wherein producing a glass preformcomprises the steps of: heating an intermediate preform so as to softenone region thereof; elongating the intermediate preform by submittingthe intermediate preform to a traction; detecting, during the step ofelongating, the preform diameter in at least one measuring point alongthe intermediate preform; controlling the step of elongating on thebasis of the detected diameter; measuring, during the step ofelongating, at least a geometrical parameter of the preform, thegeometrical parameter being different than the at least one measuringpoint; and varying, during the step of elongating, said at least onemeasuring point according to the measured geometrical parameter.
 20. Anapparatus for elongating an optical fiber preform, comprising: amonitoring device for obtaining information on geometrical parameters ofthe preform being elongated, said monitoring device comprising an imagecapturing device for obtaining a profile of at least a portion of asoftened region of the preform, and a processing device for analyzingthe profile and extracting information on the preform geometricalparameters; and a control device for controlling at least a location ofa measuring point on the preform using the preform geometricalparameters information, wherein the geometrical parameters informationis different than the location of the measuring point.
 21. An apparatusfor elongating an optical fiber preform, the apparatus comprising: meansfor elongating the preform; means for determining the preform diameterin at least one measuring point along the preform; means for controllingthe elongation of the preform on the basis of the determined diameter;means for measuring at least a geometrical parameter of the preform, thegeometrical parameter being different than the at least one measuringpoint; and means for controlling the position of said diameter measuringpoint according to the measured geometrical parameter.